Oil Pressure Mystery: Stephen's GMC Envoy

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Dear Car Talk

Apr 23, 2015 (Archives)

5 Comments

oil, warning lights

Dear Car Talk:

I have a 2006 GMC Envoy XL with 227,000 miles. After sitting in the garage overnight, when I start it, the oil pressure gauge will drop, and the oil warning lights come on. If I leave it in park and keep the engine revved up, then the oil pressure will go up. Two blocks from the house is a stoplight, and I have to put it in neutral and rev the engine, or the oil pressure drops and warning lights come on. After about 10 minutes of driving, the oil pressure still drops some at stoplights, but not low enough to cause warning lights to come on. Am I damaging the engine by continuing to drive? Why does it have oil pressure after it's warmed up but not after sitting overnight?

-- Stephen

Beats me, Stephen. Normally, oil pressure is higher when you first start your car, because the oil is cold and more viscous. So why the pressure would start low and then improve as you drive is a mystery. 

Yeah, I know, that's why you wrote to me. 

If the reading on your gauge is accurate, then yes, you are damaging the engine by driving it with low oil pressure. So the first thing you need to do is find out if your oil pressure really is low, or if you're getting an incorrect reading.

You do that by taking the gauge out of the equation. 

Your mechanic can do that for you. You'll leave the truck with him overnight. In the morning, he'll put his own oil gauge on it, and start it up. If his gauge tells him that your oil pressure is exactly what it's supposed to be, then he'll know that either your truck's oil gauge or your oil pressure sending unit is no good.

It's more likely to be the sending unit. That's a little sensor that plugs into the side of the engine, reads the pressure and sends that information to the gauge and the idiot lights on the dashboard. A bad sending unit would be the best-case scenario, Stephen; a sending unit is cheap, and very easy to replace. 

So that would be good news for both of us: Good for you, because it means you haven't been harming your truck all this time, and you'll have a nice, easy, cheap fix. Good for me, because then you won't write back again and make me hurt myself by thinking harder. I'll keep my fingers crossed for us. 

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oil, warning lights

• 5 comments
• Car Talk

bluewater • 2 days ago

I am no mechanic but I agree with you. If it were something else like the oil pump I think the engine will start to make a lot of noise if oil wasn't getting to certain parts, like the valves and lifters. I had the same thing happen on my truck. Except the oil pressure gauge stopped working all together. I could tell oil was circulating because the engine didn't sound different and the oil level on the dipstick changed when I ran the engine. I would believe it would read "full" with the motor running if were the pump. Under these circumstances, I'd just change out the Sending U nit myself before taking it to a mechanic.and see if I could save a few bucks that way.

Lee_T bluewater • 2 days ago

Correct! One of the benefits of engines with hydraulic lifters is that they act as an audible alarm if there's low oil pressure.

MartyB • 3 hours ago

The 4200 6 in the Envoy uses 7 quarts of oil. Probably the last oil change the guy put in 5 quarts. The last schlock who changed my oil put in 5 quarts and my Envoy did about the same thing. I had the oil changed again with 7 quarts and no oil light at start up

Gigem85 • a day ago

We were moving to a new town. I was driving the U-Haul and my wife was driving her car. When we pulled in the driveway she told me her car started to run a little hot. I saw water leading from the water pump so we took it to the dealer the next day.

They replaced the water pump and the harmonic balancer. When we picked up the car the oil pressure was less than 10 psi when it was normally 40 psi so we left it there. They replaced the sending unit for free but it still showed 10 psi. (they showed me the old sending unit). They called later and said it was probably bad bearings since the car had 90,000 miles on it. I told them it was fine when we brought it in.

Long story shorter, they replaced the lower end bearings for free but we still had low oil pressure. They said they couldn't do any more and we would need a new engine.

We bought a new engine and had a different mechanic shop install it. When they started it up it had 10 psi on the gauge. The mechanic installed his pressure gauge and it was reading 70 psi. He replaced the "new" sending unit and the car's gauge was reading correctly. He said sometimes even new sending units might be bad.

I don't know why the dealership mechanic didn't check the oil pressure with his own gauge before tearing into the bottom of the car. Dumb on their part. Stupid on my part to not take my car to another mechanic for a second opinion. But I would think a dealership would have qualified mechanics.

cwatkin • 2 days ago

I was driving my 2000 Chevy S-10 one day and noticed zero oil pressure. I stopped immediately and checked the level and looked for leaks. One thing I noticed when I had the hood open was like 20lbs of packrat debris piled in the engine bay. I removed all the sticks and leaves within reach and threw it off to the side of the road and noticed some chewed wires. Since the engine sounded fine before I shut it off, I suspected a wire to the sending unit was broken. I fired it up and drove it the rest of the day this way. I got home and soldiered some broken wires together and my readings returned to normal. I also wired up some rat baits under the hood.